1. Field of the Invention
The present invention relates to a method for evaluating a fastening state of a threaded joint that is used as a joint of pipes or tubes such as Oil Country Tubular Goods (OCTG) with a high degree of accuracy not only during being fastened but also after being fastened and a method for fastening a threaded joint of pipes or tubes using the evaluating method. Hereinafter, “pipes or tubes” are referred to as “pipes” when deemed appropriate.
2. Description of the Related Art
Conventionally, as a joint for OCTG, a threaded joint has been widely used. FIG. 1 is an axial directional cross sectional view that schematically illustrates a general structure of a threaded joint. As shown in FIG. 1, a threaded joint 100 is provided with a pin 1 having an external thread part 11, a metal seal part 12, a shoulder part 13 on an outer peripheral surface, and a box 2 having an internal thread part 21, a metal seal part 22, and a shoulder part 23 corresponding to each part of the pin 1 on an inner peripheral surface and being fastened with the pin 1.
The external thread part 11 and the internal thread part 21 (hereinafter, these parts are generally named as “thread parts 11, 21”) are screwed with each other so as to effect a function for fastening the pin 1 and the box 2. The external diameter of the metal seal part 12 is slightly larger than the internal diameter of the metal seal part 22 (this difference is referred to as “an interference margin”), and when the pin 1 is fastened with the box 2, due to the interference margin, a surface pressure is generated on a contact region between the both metal seal parts 12, 22 and due to this contact surface pressure, a function to sufficiently hold an air leakage efficiency of the threaded joint 100 is effected. The shoulder parts 13, 23 effect a function to prevent a high contact surface pressure such that an excess plastic transformation is generated from being generated on the metal seal parts 12, 22 and secure sufficient screwing amount so as to ascertain fastening of the threaded joint 100. Further, not only on the metal seal parts 12, 22 but also on the thread parts 11, 21, the threaded joint 100 may have the same interference margin as the metal seal parts 12, 22 in order to secure screwing of the thread parts 11, 21 so that they are not easily loosened. In this case, the shoulder parts 13, 23 also effect a function to limit the interference margins of the thread parts 11, 21 into a safe area so as to prevent an excess stress on the box 2.
As a method for evaluating a fastening state of a threaded joint having the above-described structure, conventionally, a method for monitoring change of a torque to be generated when fastening a threaded joint has been widely used (for example, refer to Japanese Patent Application Laid-Open No. 10-267175). FIG. 2 is an explanatory view for explaining a conventional method for evaluating a fastening state of a threaded joint. As shown in FIG. 2, as fastening of the threaded joint has been progressed in series, due to a frictional resistance due to interference of the thread parts 11, 21 and interference of the metal seal parts 12, 22, a torque is generated. Then, due to abutting of the shoulder parts 13, 23, the torque rapidly rises. Conventionally, good and bad of the fastening state of the threaded joint is determined by monitoring this change of the torque by an operator. In other words, in the case that the torque rises more than a predetermined threshold value, judging that the shoulder parts 13, 23 abut against with each other, it is determined that the fastening of the threaded joint 100 has been sufficiently completed.
However, according to the conventional evaluating method shown in FIG. 2, the fact that the thread parts 11, 21 interfere with each other, the metal seal parts 12, 22 interfere with each other, and the shoulder parts 13, 23 abut against with each other in face is not evaluated by measuring any physical amount independently and respectively. This is absolutely a method based on a past empirical rule such that a torque is generated because respective parts adhere tightly (interfere or abut) with each other. It is true that a torque is generated when respective parts adhere tightly (interfere or abut) with each other, however, a large torque is also generated due to other case, for example, when the thread parts 11, 21 are burnt or the like. Therefore, only by monitoring change of the torque, it is difficult to evaluate the fastening state with a high degree of accuracy.
In addition, the conventional evaluating method shown in FIG. 2 is restricted such that it is necessary to continuously monitor a torque in a process for fastening the threaded joint (in the middle of fastening a pin and a box while they are relatively moving). In other words, the conventional evaluating method is restricted such that the fastening state cannot be evaluated when the pin and the box stand still after fastening them.